This invention pertains to flowmeters, particularly flowmeters wherein a sonic signal is used to determine either the density or pressure of a fluid.
Numerous prior art flowmeters determine the density of a fluid by traversing a sonic signal from a transmitting transducer through a fluid stream to a receiving transducer and using the received energy as a measure of the fluid density. In these flowmeters it is known that a voltage drop across the receiving transducer is a linear function of the acoustical impedance (.rho.c) of the fluid where .rho. is the density of the fluid and c is the speed of sound in the fluid. It is also known that the acoustical impedance .rho.c is directly proportional to the pressure P exerted by the fluid and inversely proportional to the square root of the absolute temperature T of the fluid.
In these prior art flowmeters as basically described, the sonic, or acoustic, signal is generally reflected back and forth between the transducers, resulting in the formation of a standing wave pattern. The reflection may be caused by such factors as the dissimilar properties of the air or other medium and the transducers, transducer mounting techniques, and the preferred facing-orientation of the transducers. Although the sonic signal may have been transmitted at a relatively constant frequency, the standing wave superimposed thereon has peaks, nulls and troughs. As a result, the energy received at the receiving transducer depends to a significant degree on whether the receiving transducer is looking at any one moment at a standing wave peak, a trough, or at some point in between.
Various attempts have been made to negate the effect of standing waves in density-determining flowmeters. In some flowmeters pulse transmission may be employed so long as the pulse length is made less than the propagation time for the pulse to travel from the transmitting transducer to the receiving transducer and be reflected back. In a continuous wave transmission mode, such as in the illustrated embodiment of the invention, the standing wave may be partially negated by matching the transducers as closely as possible to the fluid impedance; by slightly tilting the transducers so that any reflected energy will not be returned to the other transducer; by placing absorption materials on the walls of the flowmeter which define the fluid stream and house the transducers; and, by placing a coupling material on the faces of the transducers. However, these techniques have not eliminated the standing wave influence to the degree required in many operating environments.
As noted above, the velocity of sound in a fluid is proportional to the fluid temperature. Hence, when the fluid temperature changes, the standing wave pattern shifts. This standing wave shift is particularly noticeable in some operating environments where the temperature of the fluid may change approximately by a factor of 2. A prime example is the air intake of an automotive internal combustion engine where the temperature ranges from approximately 220.degree. Kelvin to the neighborhood of 400.degree. Kelvin. Over such a broad temperature range the acoustic velocity in thefluid may change by as much as 33%.
Sonic flowmeters typically transmit frequencies approximately 150 KHz., the optimum frequency being based on a number of factors including signal coupling and absorption-attenuation characteristics. At these transmitted frequencies the resultant transmitted acoustic wavelengths are fairly small. Accordingly, the wavelengths of the standing wave pattern are even smaller--about half that of the transmitted wave frequency.
With these small wavelengths and over these temperature ranges, there is a change in the number of standing wave wavelengths--generally a change of 8 to 12 wavelengths--occuring between the transducers. Hence, as the temperature changes and the standing wave shifts, there is no assurance that the receiving transducer will consistently see a standing wave peak. As the standing wave seen by the receiving transducer shifts, the relative energy received varies accordingly. Hence, a density-measuring sonic flowmeter operating at a fixed frequency is subject to considerable inaccuracy due to the temperature dependency of the standing wave.
Therefore, it is an object of this invention to achieve an accurate determination of fluid density in a sonic flowmeter by eliminating the temperature dependent nature of the relative energy measurement.
In flowmeters of this type which also measure the fluid temperature by detecting the transit time of the sonic signal between the two transducers (by comparing the phase of the received signal to the phase of the transmitted signal), an ambiguity occurs in the transit time measurement. This ambiguity results from the significant propagation delay time of the transmitted signal as compared to the short time period for the transmitted acoustic frequency.
Therefore, another object of this invention is the elimination of the ambiguity involved in the fluid temperature measurement when using a flowmeter which detects a phase shift due to a propagation delay time as a sonic signal traverses the fluid.
One of the advantages of the invention is the utilization of a single sensor for the measurement of volumetric flow, fluid density, and/or fluid temperature, such measurements being spacially made in the same region.
Another advantage of the structure of the invention is the realization of substantial cost savings by using a single sensor to perform measurements heretofore performed by a plurality of sensors.